Nutrition

See Also

Description

Wheat (Triticum spp.) is a grass that is cultivated worldwide. Globally, it is the most important human food grain and ranks second in total production as a cereal crop behind maize; the third being rice. Wheat grain is a staple food used to make flour for leavened, flat and steamed breads; cookies, cakes, pasta, noodles and couscous; and for fermentation to make beer, alcohol, vodka or bio-fuel. The husk of the grain, separated when milling white flour, is bran. Wheat is planted to a limited extent as a forage crop for livestock and the straw can be used as a ruminant fodder component or as a construction material for roofing thatch.

History

Probably the first cereal to be domesticated,[9] the wheat originated in southwest Asia in the area known as the Fertile Crescent. The earliest archaeological evidence for wheat cultivation comes from the Levant, Syria and Turkey. Around 10,000 years ago,[10] wild einkorn and emmer wheat were domesticated as part of the origins of agriculture in the fertile crescent. Cultivation and repeated harvesting and sowing of the grains of wild grasses led to the selection of mutant forms with tough ears which remained attached to the ear during the harvest process, and larger grains. Selection for these traits is an important part of crop domestication). Because of the loss of seed dispersal mechanisms, domesticated wheats cannot survive in the wild.

The cultivation of wheat began to spread beyond the Fertile Crescent during the Neolithic period. By 5,000 B.P., wheat had reached Ethiopia, India, Ireland and Spain. A millennium later it reached China.[11] Agricultural cultivation using horse collar leveraged plows (3000 years B.P.) increased cereal grain productivity yields, as did the use of seed drills which replaced broadcasting sowing of seed in the 18th century. Yields of wheat continued to increase, as new land came under cultivation and with improved agricultural husbandry involving the use of fertilizers, threshing machines and reaping machines (the 'combine harvester'), tractor-draw cultivators and planters, and better varieties (see green revolution and Norin 10 wheat. With population growth rates falling, while yields continue to rise, the acreage devoted to wheat may now begin to decline for the first time in modern human history.[12][9]

Genetics and breeding

Wheat genetics is more complicated than that of most other domesticated species. Some wheat species occur as stable polyploids, having more than two sets of diploid chromosomes.

  • Einkorn wheat (T. monococcum) is diploid (2x chromosomes).
  • Most tetraploid wheats (e.g. emmer and durum wheat) are derived from wild emmer, T. dicoccoides. Wild emmer is the result of a hybridization between two diploid wild grasses, T. urartu and a wild goatgrass such as Aegilops searsii or Ae. speltoides. The hybridization that formed wild emmer occurred in the wild, long before domestication.
  • Hexaploid wheats evolved in farmers' fields. Either emmer or durum wheat hybridized with yet another wild diploid grass (Aegilops tauschii) to make the hexaploid (6x chromosomes) wheats, spelt wheat and bread wheat.

Heterosis or hybrid vigor (as in the familiar F1 hybrids of maize) occurs in common (hexaploid) wheat, but it is difficult to produce seed of hybrid cultivars on a commercial scale as is done with maize because wheat flowers are complete and normally self-pollinate. Commercial hybrid wheat seed has been produced using chemical hybridizing agents, plant growth regulators that selectively interfere with pollen development, or naturally occurring cytoplasmic male sterility systems. Hybrid wheat has been a limited commercially success, in Europe (particularly France), the USA and South Africa.F1 hybrid wheat cultivars should not be confused with standard method of breeding inbred wheat cultivars by crossing two lines using hand emasculation, then selfing or inbreeding the progeny many (ten or more) generations before release selections are identified to released as a variety or cultivar.

All four wild species of wheat, and in the domesticated einkorn, emmer and spelt wheats are hulled (in German, Spelzweizen). This more primitive morphology consists of toughened glumes that tightly enclose the grains, and (in domesticated wheats) a semi-brittle rachis that breaks easily on threshing. The result is that when threshed, the wheat ear breaks up into spikelets. To obtain the grain, further processing, such as milling or pounding, is needed to remove the hulls or husks. In contrast, in free-threshing (or naked) forms such as durum wheat and common wheat, the glumes are fragile and the rachis tough. On threshing, the chaff breaks up, releasing the grains. Hulled wheats are often stored as spikelets because the toughened glumes give good protection against pests of stored grain.

Naming

There are many taxonomic classification systems used for wheat species, discussed in a separate article on Wheat taxonomy. It is good to keep in mind that the name of a wheat species from one information source may not be the name of a wheat species in another.

Within a species, wheat cultivars are further classified by growing season, such as winter wheat vs. spring wheat[9], by gluten content, such as hard wheat (high protein content) vs. soft wheat (high starch content), or by grain color (red, white or amber). [edit]

Major cultivated species of wheat

  • Common Wheat or Bread wheat - (T. aestivum) A hexaploid species that is the most widely cultivated in the world.
  • Durum - (T. durum) The only tetraploid form of wheat widely used today, and the second most widely cultivated wheat today.
  • Einkorn - (T. monococcum) A diploid species with wild and cultivated variants. One of the earliest cultivated, but rarely planted today.
  • Emmer - (T. dicoccon) A tetraploid species, cultivated in ancient times but no longer in widespread use.
  • Spelt - (T. spelta) Another hexaploid species cultivated in limited quantities.
  • Kamut� or QK-77 - (T. polonicum or T. durum) A trademarked tetraploid cultivar grown in small quantities that is extensively marketed. Originally from the Middle East

Harvested wheat grain is classified according to grain properties (see below) for the purposes of the commodities market. Wheat buyers use the classifications to help determine which wheat to purchase as each class has special uses. Wheat producers determine which classes of wheat are the most profitable to cultivate with this system.

Wheat is widely cultivated as a cash crop because it produces a good yield per unit area, grows well in a temperate climate even with a moderately short growing season, and yields a versatile, high-quality flour that is widely used in baking. Most breads are made with wheat flour, even many breads named for the other grains they contain, including most rye and oat breads. Many other popular foods are made from wheat flour as well, resulting in a large demand for the grain even in economies with a significant food surplus.

1997 global per capita wheat consumption was 101 kg, led by Denmark at 623 kg.

Crop development

Crop management decisions require the knowledge of stage of development of the crop. In particular, spring fertilizers applications, herbicides, fungicides, growth regulators are typically applied at specific stages of plant development.

For example, current recommendations often indicate the second application of nitrogen be done when the ear (not visible at this stage) is about 1 cm in size (Z31 on Zadoks scale). Knowledge of stages is also interesting to identify periods of higher risk, in terms of climate. For example, the me�osis stage is extremely susceptible to low temperatures (under 4 �C) or high temperatures (over 25 �C). Farmers also benefit from knowing when the flag leaf (last leaf) appears as this leaf represents about 75% of photosynthesis reactions during the grain filling period and as such should be preserved from disease or insect attacks to ensure a good yield.

Several systems exist to identify crop stages, with the Feekes and Zadoks scales being the most widely used. Each scale is a standard system which describes successive stages reached by the crop during the agricultural season.

Diseases

Estimates of the amount of wheat production lost owing to plant diseases vary between 10-25% in Missouri[2]. A wide range of organisms infect wheat, of which the most important are viruses and fungi.

Pests

Wheat is used as a food plant by the larvae of some Lepidoptera species including The Flame, Rustic Shoulder-knot, Setaceous Hebrew Character and Turnip Moth.

  • Durum � Very hard, translucent, light colored grain used to make semolina flour for pasta.
  • Hard Red Spring � Hard, brownish, high protein wheat used for bread and hard baked goods.
  • Hard Red Winter � Hard, brownish, very high protein wheat used for bread, hard baked goods and as an adjunct in other flours to increase protein.
  • Soft Red Winter � Soft, brownish, medium protein wheat used for bread.
  • Hard White � Hard, light colored, opaque, chalky, medium protein wheat planted in dry, temperate areas. Used for bread and brewing
  • Soft White � Soft, light colored, very low protein wheat grown in temperate moist areas. Used for bread.

Hard wheats are harder to process and red wheats may need bleaching. Therefore, soft and white wheats usually command higher prices than hard and red wheats on the commodities market.

Much of the following text is taken from the Household Cyclopedia of 1881:

Wheat may be classed under two principal divisions, though each of these admits of several subdivisions. The first is composed of all the varieties of red wheat. The second division comprehends the whole varieties of white wheat, which again may be arranged under two distinct heads, namely, thick-chaffed and thin-chaffed.

Thick-chaffed wheat varieties were the most widely used before 1799, as they generally make the best quality flour, and in dry seasons, equal the yields of thin-chaffed varieties. However, thick-chaffed varieties are particularly susceptible to mildew, while thin-chaffed varieties are quite hardy and in general are more resistant to mildew. Consequently, a widespread outbreak of mildew in 1799 began a gradual decline in the popularity of thick-chaffed varieties.

Lectins

Contains the lectin [Wheat Germ Agglutinin]? (WGA).

Links

Attribution